Cognitive symptoms of Parkinson's disease are emerging as an enormous public health problem. Up to 80% of PD patients will suffer debilitating cognitive symptoms in the course of their disease. In PD patients, cognitive impairments predict a malignant disease course leading to loss of employment, independence, driving deficits, nursing home placement, and death. Because PD is strongly associated with aging, this problem will surge as our society ages. There are few treatments that improve PD-related cognitive symptoms. Thus there is a critical need to develop new, mechanistic treatments for cognitive symptoms of PD. A challenge in developing new treatments is that there is a knowledge gap about the mechanism of PD-related cognitive symptoms. Cognitive deficits in PD patients include impaired working memory, attention, reasoning, planning, and timing. One elementary cognitive task in which PD patients are reliably impaired is interval timing. In this task, subjects are presented with a stimulus, and estimate its duration over several seconds. Interval timing is an ideal window into cognition in PD because this task depends on dopamine and can be readily studied in animal models. Elucidating the neural circuitry of interval timing could help close the knowledge gap about cognitive dysfunction in PD. Our preliminary data strongly implicate D1-type dopamine receptors on pyramidal neurons in the prefrontal area of the cerebral cortex in interval timing. However, it is unclear precisely how prefrontal neurons influence interval timing. Here we combine highly selective and specific techniques such as optogenetics, focal drug infusions, and neuronal ensemble recordings to systematically interrogate the neural activity of prefrontal D1 neurons in great detail, and to map the projections of these neurons. In Aim 1, we determine how prefrontal D1 neurons control interval timing. In Aim 2, we determine which projections of prefrontal D1 neurons control interval timing. Finally, in Aim 3 we rescue interval timing deficits in animal models of PD by stimulating prefrontal D1 neurons and their projections. This work will identify key drivers of a cognitive process impaired in PD patients. Our findings could link a brain region and a receptor system to cognitive processes impaired in PD, and could spur development of targeted pharmacological, genetic, or brain-stimulation therapies. Insights from this work could have relevance for PD as well as for other diseases involving prefrontal dopamine circuits, such as schizophrenia, ADHD, addiction, and Huntington's disease.